This invention relates to glass compositions and is concerned with the provision of improved glasses having softening temperatures (E.sub.W) between 596 and 634.degree. C. and working temperatures (V.sub.A) between 792 and 863.degree. C. The softening temperature (E.sub.W) is that at which the viscosity of the glass is 10.sup.7.6 poise and the working temperature (V.sub.A) is that at which the viscosity of the glass is 10.sup.4 poise. The transformation temperature (Tg) of the glass, i.e. the temperature at which the glass has a viscosity of 10.sup.13.5 poise, may be between 428 and 488.degree. C. and the coefficient of thermal expansion (.alpha.) of the glass between 20.degree. and 300.degree. C. is preferably between 80.1 and 110.2 .times. 10.sup.-.sup.7 /.degree. C.
The requirement for such glass compositions has arisen in the manufacture of electrical circuit components in which sensitive elements such as diodes are hermetically sealed within glass capsules or in which magnetically operated contact switches are fixed within glass containers and operate in a closed inert atmosphere. The manufacture of such components has led in the last decade to the development of special glasses, which are sometimes known as reed switch glasses because of their particular commercial application.
These glasses have been developed as a result of the requirement for bonding electrical conductors, such as contact switches, to glass in a defined and controllable atmosphere (of an inert or reducing gas) or in a vacuum without using the gaseous heating techniques which are normally employed for the hot-working of glass. The use of electric coil heating, and in particular beam heating with quartz-iodine lamps concentrated by means of gold reflectors, which has become important in recent years, has given rise to a group of infra-red-absorbing glasses, the common characteristic of which is a blue-green colour as a result of the divalent iron content in the form of the FeO necessary to provide the IR-absorption.
A further general aim in the development of this group of glasses has been to keep the temperature necessary for hot working and glass-metal bonding as low as possible. Besides purely economic points of view, such as energy consumption, working life of the radiating system, and speed of the bonding process, vaporisation of constituents of the glass during the bonding process is also important. Condensates of easily evaporable oxides such as for example B.sub.2 O.sub.3 and K.sub.2 0, can arise in the production area, and this is undesirable in view of the possible effect of such condensates on the operation and effective life of the controls. The evaporation of oxides such as these during the bonding and melting process depends on the glass composition, as well as on the melting temperature.
With these main points of view in mind a series of glasses containing FeO have been developed and put into use; these can be divided into three groups according to composition and physical behaviour, as shown in the following table:
______________________________________ Group Group C Group B Alkali-Silicate; A Alkali- lead-free Alkali-lead- silicate; poor evapor- silicates lead-free ation ______________________________________ Density 3.05 - 3.15 2.52 - 2.56 2.51 - 2.65 (g/cm.sup.3) Tg 430 - 440 440 - 460 470 - 490 E.sub.W 620 - 630 620 - 660 660 - 680 V.sub.A 970 - 980 960 - 980 950 - 1000 ______________________________________
While group A glass was developed from a lead glass (PbO-content 26 - 30 weight -%) already internationally known and used for a long time by the introduction of FeO, Group B glass is a special development for this application. Here, the aim has been to produce a lead-free glass of which the temperatures Tg, E.sub.W and V.sub.A do not differ to any great extent from the glasses of Group A. With the decrease in the lead content, the effects of reduction during hot working of the glass can be avoided. Besides the decrease in the lead content, an attempt was made with glasses of Group C to reduce the content of readily vaporised oxides and consequently to avoid the formation of condensates in the switch room.
Apart from the differences described above between these three groups of glass, there are only small or technically hardly significant differences between the temperatures, which have to be utilized for hot working of the glasses in the production of electronic components. The temperature at which a viscosity of 10.sup.4 poise is obtained can be taken as the prime characteristic for this. From this it follows that the hitherto known reed glasses can be regarded as practically equivalent as regards the working temperature and in connection with this, the rate of production, as described in German Pat. Specification No. 2,116,155.